An alternator for automobiles is a three-phase alternating current generator, in which electric power is produced from torque obtained by an engine to be fed to a battery, and an alternator diode functions to rectify three-phase alternating current produced by the generator into direct current in order to feed the current to the battery. An alternator diode comprises a silicon (Si) chip having a rectifying function, a lead and a base, which have a current-carrying function, and solder, which joins these elements together, and is structured to fill a resin, such as silicon rubber, etc., inside the base.
When an alternator operates, large current flows through an alternator diode, so that a silicon chip generates heat due to Joule heating and the silicon chip, surrounding solder, a lead and a base become equal to or higher than 200° C. at the highest. When the alternator stops, electric current is not supplied and the alternator diode is cooled to an ambient temperature. Since the alternator repetitively operates and stops over a long term, the alternator diode repetitively undergoes expansion due to heating and contraction due to cooling. At this time, the silicon chip, the lead, and the base are different in coefficient of linear expansion to generate differences in volume of thermal deformation, so that thermal stress is generated in solder, which joins these elements together, to cause a fear that the solder undergoes fatigue failure because of such thermal stress.
Therefore, there is proposed a diode shown in, for example, U.S. Pat. No. 4,349,831, which comprises buffer layers provided between a silicon chip and a lead and between the silicon chip and a base and formed from a material, which is larger in coefficient of linear expansion than that of silicon and smaller than materials of the lead and the base.